Reds and Greens colormap starting with black

Community matplotlib-users
1

Hi matplotlib users,

I was wondering if anyone knows a way to modify the Reds and Greens
colormap such that the lower end of the colormaps is not a mixture of white
and red or green, but black and red or green respectively.

I tried to do this by making the image RGB, with my data only in the red or
green channel, but matplotlib only supports int8 but my images have higher
dynamic range, and they lose resolution if I cast them from int16 to int8.
Here's the code:

def color(img):
    x, y = img.shape
    c = np.zeros((x, y, 3), dtype=np.uint8)
    c[:, :, 0] = img
    return c

Thank you and best regards,
Kesavan
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2

Kesavan,

Check out the BlueRed1 colormap near the bottom of
https://matplotlib.org/gallery/images_contours_and_fields/custom_cmap.html?highlight=linearsegmentedcolormap.
  It is shown in the upper-left panel of the figure at the very bottom.
It is not exactly what you are asking for, but it illustrates how to
make simple colormaps that shade into black.

Alternatively, for a good colormap tool, see
GitHub - matplotlib/viscm: A tool for visualizing and designing colormaps using colorspacious and matplotlib. You should be able to get a much
better result with this than with simple linear ramps in RGB space.

Eric

···

On 2017/12/19 9:12 PM, Kesavan Subburam wrote:

Hi matplotlib users,

I was wondering if anyone knows a way to modify the Reds and Greens
colormap such that the lower end of the colormaps is not a mixture of
white and red or green, but black and red or green respectively.

I tried to do this by making the image RGB, with my data only in the red
or green channel, but matplotlib only supports int8 but my images have
higher dynamic range, and they lose resolution if I cast them from int16
to int8. Here's the code:

def color(img):
??? x, y = img.shape
??? c = np.zeros((x, y, 3), dtype=np.uint8)
??? c[:, :, 0] = img
??? return c

Thank you and best regards,
Kesavan

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